This invention relates to eyepieces suitable for use with either astronomical or terrestrial telescopes, and more particularly to eyepieces having an extremely wide field of view with essentially perfect correction of all monochromatic and chromatic aberrations.
In the ideal case, an eyepiece should present to the eye of the observer a virtual image focused at a convenient distance (typically at a distance of one meter) with uniform sharpness of the image over the entire field of view. In addition, all of the ray bundles from objects in various parts of the field should, in the absence of the observer's eye, cross at a single point on the optical axis, said point being the exit pupil of the system. In addition, for maximum utilization of the observer's eye, the apparent field should be about 90.degree. in extent.
In the prior art, these goals have substantially failed to be met for a variety of reasons. In eyepieces consisting wholly of lens elements located between the focal plane of the telescope and the observer, such as U.S. Pat. No. 3,384,434, the field of view is severely limited, in practical cases to about 60.degree. to 70.degree., by the large amount of astigmatism inherent in those designs. In the case of U.S. Pat. No. 4,286,844, where a negative lens group is located on the far side of the focal plane from the observer, the astigmatism is better corrected and the design is capable of covering a field of about 90.degree.--but in this case there is such a large amount of spherical aberration of the exit pupil that, for many applications, especially for daytime use when the iris of the user's eye is more constricted than at nighttime, it is impossible for light from all parts of the field of view to enter the user's eye at the same time. What the user sees is a wide field, portions of which are completely black while other portions are illuminated. As the user moves his or her eye from side-to-side, different portions of the field become alternately darkened and illuminated--but at no time is the whole field visible at once. This defect is known as the "kidney bean" effect, because the missing, darkened portions of the field are delimited by a boundary of roughly the shape of a kidney bean.